Koudougou Station TEC’s Variability Seasonal Anomalies Analysis During Fluctuating Events Over Solar Cycle 24

This paper deals with the analysis of anomalies on variations of total electron content (TEC) during fluctuating geomagnetic events through solar cycle 24 at Koudougou station (lat 12°15'N; long:-2°20'E). Study of anomalies was done by comparing the variations of TEC during solstice and equinox seasons and variations of TEC at equinoxes to those at solstices. Comparison was made taking into account solar cycle phases and seasons influences. Equinoctial asymmetry is recorded at ascending and maximum phases. Winter anomaly is observed all night (0000 LT-04 00 LT and 2000 LT-2300 LT) at solar maximum and at all times during the ascending phase, except around 0600 LT and 1800 LT. Semi-annual asymmetry is observed during all phases of solar cycle 24 during fluctuating events.


Introduction
Ionosphere is the ionized region of Earth's upper atmosphere located between about 50 km and 600 km. The density of plasma in ionosphere presents important variations in time (sunspot cycle, seasons, diurnal) and space (altitude, latitude, longitude) which results in a modification of ionospheric parameters such as total electron content (TEC). TEC is the number of free electrons in the ionized plasma contained in imaginary tube of cross-section, the ends of which are bounded by orbiting satellite and ground receiver (Kersley et al. 2004). Density of F-region contributes mostly to maximum TEC that affects mainly radio waves. In the low latitude equatorial ionosphere in the F-region, ionization density distribution is characterized by a trough at equator and double peaks on either side of equator, almost at about ± 15° magnetic latitude are called the Equatorial Ionosphere Anomaly (EIA) peaks. TEC in the upper atmosphere plays a crucial role in determining range delays by electromagnetic signals as they cross the ionosphere (Rao et al. 2013). Therefore, it is important to study the TEC, as well as possible anomalies that may appear during the variations of this parameter (TEC). Observations have shown that in the low latitude region of ionosphere' F layer, electron density is subject to several anomalies among which we have: (1) winter anomaly, (2) annual anomaly, (3) semi-annual anomaly and (4) equinoctial asymmetry.
Winter anomaly is a phenomenon characterized by higher values of electron density in winter than in summer during the day; it is an anomaly that usually disappears at night . Several mechanisms have been suggested explain it. For some authors, it could be due to changes in the composition of neutral atmosphere of the F region (Rishbeth et Edwards 1989;M. R. Torr et Torr 1973;M. Torr, Torr, et Hinteregger 1980). According to Johnson (1964) these changes could occasion an increase in the ratio due to convection of oxygen atoms from summer hemisphere to winter hemisphere. For others such as, Roble et al (1977) and Roble (1983) this anomalous variation in the neutral composition of F region could also be associated with seasonal variations in thermospheric winds.
Semi-annual anomaly is a phenomenon during which electron density is higher at equinoxes than at solstices. Titheridge (1973) and Rishbeth et al (2000b) showed that the change in ratio of atomic oxygen to molecular nitrogen concentrations ( ) causes the semi-annual anomaly. According to Ma et al. (2003) equatorial electrojet currents fluctuations, due to semiannual variations of diurnal tide in lower thermosphere, are the cause of variation in amplitude of equatorial ionospheric anomaly by fountain effect; which is the origin of semi-annual NmF2 anomaly in the low latitudes. Some authors attribute semi-annual variations in density of neutral components and F2 layer electrons ( 2) to solar winds influence (Lal 1992;1998). Ionospheric equinoctial asymmetry is difference in ionization between the two equinoxes Bailey, Su, and Oyama 2000;Chen et al. 2012;Kawamura et al. 2002;Ouattara et al. 2017;Ren et al. 2011), despite the fact that the zenith solar angle has almost the same characteristics during these two periods (Spring and autumn).
Several authors' works on morphological characteristics of the TEC such as diurnal, monthly, seasonal, latitudinal and solar activity variation, has improved the understanding of this parameter. We have among others Walker et al (1994) Sahai et al (2007)   . In addition, TEC variations have been studied in the equatorial region of Africa; for example, Ouattara et al. (2017) and Zoundi et al. (2012) Ouattara et al. (2017) show that Niamey CODG TEC variation follows solar cycle and present semi-annual variation with equinoctial asymmetry. And Zoundi et al. (2012) show that perturbed solar events produce positive storms and only the shock event causes a peak in the TEC at noon.
The novelty of this study is that it's the first-time seasonal anomalies in TEC variations have been analyzing at Koudougou station during fluctuating activity. The present paper aims to investigate seasonal anomalies observed during fluctuating events at Koudougou station, an African Equatorial Ionization Anomaly (EIA) region station, by using TEC values computed in this GPS station. Period of investigation covers solar cycles 24, from 2008 to 2018, and concerns the four solar phases (minimum, increasing, maximum and decreasing phases).
Section 2 is devoted to materials and methods. Section 3 presents Results and discussions and section 4 is deserved to conclusion.

Data
TEC data used are from Koudougou GPS station. The receiver was provided by the Ecole Nationale de Té lé communication of Bretagne (ENST Bretagne) as part of the International Heliophysical Year (IHY) project initiated by Europe-Africa Study and Research Group (GIRGEA). This receiver has been installed at University Norbert ZONGO since November 2008. TEC data recorded cover solar cycle 24, except for the years 2008; 2018 and 2019. The geomagnetic index data and the dates of Sudden Storm Commencement (SSC) were used (P Mayaud 1973) to elaborate the pixel diagram. This diagram represents geomagnetic index evolution as a function of solar activity for each Bartels rotation (Ouattara et al. 2009). The mean sunspot number ( ) annual was used for the division of the solar cycle 24 into phases.
The geomagnetic index is deduced from K-index measured at two mid-latitude antipodal stations. This index measures the amplitude of global geomagnetic activity during 3-hour intervals normalized to geomagnetic latitude . It was introduced by Mayaud (1971) to monitor geomagnetic activity over the longest possible period. The daily average of the 8 tri-hourly values per day is noted . An SSC corresponds to an abrupt change in the geomagnetic field followed by a geomagnetic storm that lasts less than an hour. The dates of SSCs and the values of the index since 1869 are available on the ISGI website (http://isgi.unistra.fr). values are available on the OMNIWEB website: https://omniweb.gsfc.nasa.gov/form/dx1.html. Legrand and Simon (1989); and Richardson et al. (2002; elaborated geomagnetic activities' first classification from pixel diagram. They classified geomagnetic activities into four classes: (1) quiet activities associated with slow solar winds ( < 450 / ); (2) recurrent activities caused by fast solar winds from coronal holes ( > 450 / ); (3) shock activities related to shock waves due to Coronal Mass Ejections (CMEs); and (4) fluctuating activities caused by fluctuations in the Sun's neutral blade. Ouattara and Amory-Mazaudier (2009) continued, in the same dynamics, by improving this method.

Geomagnetic Activities' Classification Methods
This classification was further improved by Zerbo et al. (2012), who pushed the limits of old classification (AC) by providing clarification of solar origin of about 20% of the geomagnetic storms in addition to the 60% explained by AC. In new classification (NC), days of quiet activity, associated with slow solar winds, correspond to days when and disturbed activity to days when . Disturbed days include: i) fluctuating activity days (AFs) or fluctuating events (EFs) caused by fluctuations in the Sun's neutral blade, ii) shock events (SEs) including shock activity (SA) and cloud shock activity (CSA), and iii) recurrent event days (REs) including classical recurrent activities of AC (RAs) and Corotating Moderate Activities (CMA) due to stable corotating neutral winds.
Geomagnetic activity days selection is done using the pixel diagram ( Figure 1) proposed by Simon and Legrand (1989) and improved by Ouattara and Amory-Mazaudier (2009) who organized it in columns and rows; then they defined a color code to identify different types of geomagnetic activities. A line in the diagram corresponds to a solar rotation (27 days). The SSC dates are indicated by circles surrounding the corresponding index value. The dates of the beginning days of Bartels cycle, the legend and the year are shown on the left, right and above the pixel diagram respectively. According to pixel diagram color code, different geomagnetic activities days are selected as follows: 1) Quiet activities (QA) corresponding to days with which are represented by white and blue boxes; 2) Recurring events (RE) including: i) classical recurrent activities (RA) of AC corresponding to days with and spanning one or more Bartels rotations without SSC; these days are represented by orange, red, and/or dark red boxes on at least two successive days without SSC and on at least two solar rotations; ii) and CMA days corresponding to days with and spanning one or more Bartels rotations without SSC; these days are represented by yellow or green boxes on at least two successive days without SSC and on at least two solar rotations

3) Shock events (SE) including: i)
Classical shock activities of AC (SA) corresponding to SSC days with ; these days are represented by a set of 1, 2, or 3 days represented by orange, red, and/or olive-red boxes with SSC in phase beginning and no recurrence of SSC during 1, 2, 3, or 4 Bartels rotations; ii) and CSA days corresponding to SSC days where ; these days are represented by a set of 1, 2, or 3 days represented by yellow and green boxes with SSC at phase beginning and no recurrence of SSC during 1, 2, 3, or 4 Bartels rotations

Solar Phases Determination Criteria
Solar cycle is divided into phases according to criteria proposed by Ouattara and Amory-Mazaudier (2009). These criteria related to the annual average of sunspots number are defined as follows: i) minimum phase: ; ii) ascending phase: with greater than that of previous year; iii) maximum phase: noting that for weak solar cycles (with ) the years of maximum phase correspond to those with an index , and iv) decreasing phase: with lower than that of the previous year.  Table 2 shows results of solar cycle 24 cutting into phases.

Data Analysis Methods
The error bars placed on TEC profiles of the QA, for the qualitative analysis of FE compared to QA, are obtained by √ where is the variance given by equation 4. If δTEC is within ∓20%, the skewness is said to be insignificant; otherwise, the conclusion is that a clear skewness exists.

Results
3.1.1 Occurrence of Fluctuating Activities by Season and Solar Phase Figure 2. Variability in occurrence of fluctuating events by solar phase and season Figure 3 shows the occurrence of FEs by season and by solar phase. At solar minimum FEs occurrence decreased from spring (33%) to summer (17%) and increase in autumn (22%) and in winter (28%). In the descending phase FEs are predominant in spring season (32.65%) followed by summer season (28.57%). A low occurrence of FEs is recorded in autumn (21.77%) and winter (17%). During the maximum phase more FEs days are recorded in winter (29.91%). In spring and autumn, the number of FEs days is low with an occurrence rate of 25.64% and 23.93% respectively. Lowest occurrence of FEs is recorded in summer. During the decreasing phase, FEs days appear in a decreasing manner in winter (27.87%), spring (25.25%), summer (24.26%), and fall (22.62%). In general, EF days appear more in spring during the minimum and ascending phases; in winter during the maximum and descending phases.

Seasonal Anomalies Analysis on TEC Variations per Solar Phase
Figure 3 through Figure 6 show diurnal seasonal variations in TEC and relative deviation δTEC profiles between seasons during minimum, ascending, maximum and descending solar phases. These plots are related to study equinoctial (panel "a"), winter (panel "b") and semi-annual (panel "c") anomalies on TEC variations at Koudougou station during solar cycle 24. At solar minimum (Figure 3), the study is devoted only to semi-annual anomaly analysis because during this phase there is a lack of TEC data for FEs days of summer and autumn seasons.  Figure 4"a", We note that at the equinoxes, the TEC plot shows a trough around noon and a night peak (2200 LT) in the spring, reflecting the presence of equinoctial asymmetry on the drift and on the PRE. Furthermore, the error bars show that the ionization is, at all times, different at the two equinoxes testifying to the presence of equinoctial asymmetry. δTEC curve shows values greater than 20% all the time except at 1500 LT and 2000LT; the maximum difference between the two equinoxes is located at 0000 LT with a peak . This shows a clear presence of equinoctial asymmetry at the rising phase throughout the day. However, the asymmetry is less important around 1500 and 2000 LT.
In Figure 4"b", The TEC graphs show the presence of annual asymmetry on upward drift of plasma which results in the presence of a trough around noon only in summer. Absence of annual asymmetry on the PRE is noted because both graphs (winter, summer) have similar evolution at night. Error bars show the absence of a winter anomaly in the TEC at all times which is reflected in larger values in summer than in winter. values exceed 20% at all times except between [0100 LT -0200LT], [1000 LT -1200LT] and [2100 LT -2300LT] where the values are between ∓20%. From these observations we note the almost complete absence of winter anomaly on the TEC at the ascending phase.
In Figure 4"c", the TEC graphs show, on the one hand, the absence of semi-annual asymmetry on the upward drift which translates into the absence of ionization troughs at the solstices and equinoxes and, on the other hand, the existence of semi-annual asymmetry on the PRE which translates into the presence of a night peak only at the equinoxes. Error bars show that the ionization is different at both seasons at all times. Equinox curve is above the solstice curve all the time, testifying to the presence of semi-annual asymmetry. δTEC curve has values greater than +20% at all times. Then, there is the absence of semi-annual asymmetry on the drift, and its existence on the evening ionization peak and on the diurnal TEC variation profiles. In Figure 5"a" an absence of noon troughs at equinoxes (spring, fall) and the presence of night peaks only in spring are observed; thus, reflecting the absence of equinoctial asymmetry on the drift at equinoxes (spring, fall) and the presence of equinoctial asymmetry on the PRE. Error bars show that the ionization is, at all times, different at both equinoxes testifying the presence of equinoctial asymmetry. On the other hand, present values between before sunrise (0300 LT -0600LT) and from 1300 LT to 2300LT. Therefore, the equinoctial asymmetry on the TEC variations is considerable at all times except at the hours mentioned above during the maximum phase. Graphs in Figure 5 "b", show the absence of semi-annual asymmetry on the drift (no ionization dip in winter and summer) and the existence of this asymmetry on the PRE (night peak only in summer ( ). Winter curve is above the summer curve all the time except in the morning (0500 LT-0800 LT) and evening (1700 LT-1800 LT). Error bars show that the ionization is different at both solstices at all times. curve shows that there is a winter anomaly with values less than between 0000 LT and 0400 LT and from 2000 LT to 2300 LT. Indeed, the absence of winter anomaly is observed between 0500 LT and 0600 LT as values are greater than +20%. Thus, at solar maximum, the winter anomaly is observed on TEC variations almost all night (0000-0400 LT and from 2000-2300 LT).
Panel "c" of figure 5 shows the existence of semi-annual asymmetry on the PRE (nighttime peak observed only at equinoxes). Error bars show that the ionization is different at both seasons all the time. Equinox curve is above the solstice curve all the time, testifying to the presence of semi-annual asymmetry on the variations of TEC. Positive values of all the time confirm this observation with values above all the time indicating that the phenomenon is very important at solar maximum.

Seasonal Anomalies on the Variability of TEC in the Downward Phase
Graphs in Figure 6 "a" show the absence of equinoctial asymmetry on the drift as well as on the evening ionization peak. Error bars show that the ionization is different in the two seasons at all times. Spring curve is above the fall curve all the time, testifying the absence of the equinoctial asymmetry.
values are positive all the time and greater than +20% around 0000 LT at 0600 LT and from 2200 to 2300 LT indicating the period when the asymmetry is intense.  Figure 6 "b" the graphs show the absence of winter anomaly on and on the PRE. Error bars show that ionization is different in both seasons at all times. Winter curve is above the summer curve all the time except in the morning (0600 LT) and evening (1800 LT), testifying to the presence of the winter anomaly at the solstices except at 0600 LT and 1800 LT. values are lower than between 0000 LT and 0400 LT and from 2000 LT to 2300 LT showing that at this period the phenomenon is intense. Figure 6. "c", shows the absence of semi-annual asymmetry in and PRE. Error bars show that the ionization is different at the two seasons at all times. Equinox curve is above the solstice curve all the time, showing the presence of the semi-annual asymmetry on the variations of the TEC. δTEC curve shows values greater than +20% from 0000 LT to 0500 LT and from 1500 LT to 2300 LT showing that at this period the asymmetry is considerable. In sum, there is the absence of semi-annual asymmetry on the drift, on the evening ionization peak, and the presence of this asymmetry on the diurnal TEC variation profiles during the downward phase.

Discussion
From the analysis of possible seasonal anomalies on TEC diurnal variations during FEs, it appears that: (1) Equinoctial asymmetry is observed at the ascending phase and solar maximum. On a seasonal scale, FEs predominate in spring than in autumn during the ascending phase and at solar maximum. This suggests that on the solar phase scale, the presence of equinoctial asymmetry is not related to    suggest that the asymmetric variation of the temperature and composition of thermospheric winds would be at the origin the equinoctial asymmetry of ionospheric plasma density. In addition, the work of Ouattara et al. (2017) at Niamey station (lat. 13.3° N; long. 2.0° E) showed that the Russell McPherron mechanism is a cause of the equinoctial asymmetry of the TEC.
(2) At solar maximum the winter anomaly is present all night (0000 LT-0400 LT) and from (2000 LT-2300 LT); and during the descending phase it is observed all the time except at 0600 LT and 1800 LT. Contrary to the observation of Rishbeth et al., (2000b) which shows that the winter anomaly disappears at night. On a seasonal scale, FEs predominate in winter than in summer at maximum and at downward phases. This shows that on the scale of solar phases and seasons, the presence of anomaly is related on the one hand to the occurrence of FE days. On other hand, this anomaly could be due to changes in the composition of the neutral atmosphere of F-ionospheric region (Rishbeth et Edwards 1989;M. R. Torr et Torr 1973;M. Torr, Torr, et Hinteregger 1980). And according to Johnson (1964), these changes could occasion an increase in the ratio of ; due to convection of oxygen atoms from the summer to the winter hemisphere.
(3) Semi-annual asymmetry is observed during all phases of solar cycle 24 during fluctuating events. It is most pronounced at low and mid-latitudes (M. R. Torr et Torr 1973;Yonezawa 1971). Anomaly is related to the variation in the temperature of the Earth's upper atmosphere Yonezawa (1971). According to Mahajan (1971) and Torr and Torr (1973) it is due to semi-annual variation in the density of neutral atmosphere at low altitudes related to geomagnetic and auroral activities. On other hand, Rishbeth et al. (2000b) showed that the change in ratio of atomic oxygen to molecular nitrogen concentrations ( ) causes the semi-annual anomaly.

Conclusion
Anomalies analysis on TEC diurnal variations during FEs of solar cycle 24 at station of Koudougou, showed the presence of equinoctial asymmetry at the ascending phase and at the phase maximum. The winter anomaly is observed all night (0000 LT-0400 LT) and from 2000 LT to 2300 LT at solar maximum and all the time except around 0600 LT and 1800 LT at the ascending phase. Semi-annual asymmetry is observed during all solar phases. From the analysis of Fes occurrences by season and by solar phase, it is generally found that they appear more in spring, at minimum and ascending phases. They predominate in winter during maximum and descending phases. Equinoctial asymmetry is therefore not related to FEs occurrences, which is not the case for the winter anomaly.

Data Availability
The sunspot data used to support the findings of our study are available at: https://www.sidc.be/silso/datafiles. The geomagnetic aa index data are available at https://isgi.unistra.fr/data_download.php.

Conflicts of Interest
The authors declare no conflicts of interest.